First evidence for anomalous thick crust beneath mid-Archean western Dharwar craton

Abstract

We report an anomalous present day crustal thickness of 43–52 km beneath the 3.4–3.0 Ga mid-Archean segment of the western Dharwar craton (WDC) undisturbed by Proterozoic events. In contrast, adjoining late-Archean (2.7–2.5 Ga) eastern Dharwar craton (EDC) has a 33–40 km crustal thickness similar to the Archean global average. Considering that mineral assemblages in the central part of the WDC crust (amphibolite grade metamorphics) equilibrated at a depth of 15–20 km, we argue that the western Dharwar crust 3.0 Ga ago must have been at least 60– 80 km thick. Both segments of Dharwar craton crust exhibit Poisson’s ratio of 0.24–0.28 suggesting felsic to intermediate average crustal composition. The thickest crust beneath WDC has also underlying highvelocity thicker lithosphere compared to EDC, inferred from faster arrivals of teleseismic P and S wave. The contact between WDC and EDC is marked as gradational thinning of crust (42–36 km) from Chitradurga thrust to the western part of Closepet granite. In WDC, the crustal thickness increases in step fashion towards the oldest crustal block. These details suggest terrain accretion in Dharwar craton during 3.4 to 2.5 Ga through subduction related process